Conventional insulated electric wire for high-voltage direct current used in such applications as high-voltage wiring for television receivers and the like include those disclosed in JP-B-51-8465, JP-B-U-56-55859, and JP-B-60-38805. (The terms "JP-B" and "JP-B-U" as used herein mean an "examined Japanese patent publication" and an "examined Japanese utility model publication", respectively.)
JP-B-51-8465 discloses an insulated electric wire which comprises a conductor, a polyethylene insulation layer formed thereon, and a sheath layer formed on the outer periphery of the insulation layer. The sheath layer comprises a resin composition containing an ethylene-vinylacetate-vinylchloride terpolymer as the major component. The electric wire is excellent in flame retardation and high-temperature electrical characteristics.
JP-B-U-56-55859 discloses a high-voltage wiring for television receivers, which comprises a conductor having thereon a covering of a three-layer structure. The covering is composed of a polyethylene insulation formed on the conductor, a flame retardant resin layer composed of electron beam-irradiated polyvinylchloride formed on the outer periphery of the insulation, and a crosslinked polyethylene sheath layer formed on the outer periphery of the flame retardant resin layer.
JP-B-60-38805 discloses an insulated electric wire comprising a conductor, an insulation layer composed of a resin composition containing polyethylene as the major component formed on the conductor, and a flame retardant protective layer formed on the outer periphery of the insulation layer.
It is apparent from the above that all the conventional insulated electric wires for high-voltage direct current have such a structure in that an insulation layer composed of a polyethylene-based resin composition is provided on the conductor, and a flame retardant protective layer is further provided on the outer periphery of the insulation layer.
The conventional insulated electric wires for high-voltage direct current as described above have a problem in that the breakdown voltages of these wires drop significantly during long-term use, although the initial breakdown voltages thereof are as extremely high as about 300 kV.
Use of these conventional insulated electric wires under relatively mild electrical conditions as in conventional applications has not posed any problem. However, the electrical conditions under which insulated electric wires are used become severe. For example, in television receivers, the voltage applied to the conductor of the electric wire becomes higher due to the recent trend of increasing the size of television receiver screens, i.e., the size of cathode ray tubes. Further, because of the trend toward increase in the number of functions for users to be possessed by a single television receiver, the wiring space within the receiver is becoming small, resulting in, for example, a construction in which the high-voltage wire is very close to the grounded circuit.
Because of the above, the decrease in breakdown voltage during the long-term use of the conventional insulated electric wires is in considerable danger of leading to electrical breakdown. Hence, an improvement in this respect has become necessary.